Estimating Dissolved Phosphorus Concentrations in Surface Runoff and Leaching Water from Mineral and Organic Soils.

Losses of phosphorus (P) from agricultural soils have been long identified as one of the major causes of decreased surface water quality. This study was conducted to evaluate several environmental and agronomic soil P tests as indicators of potential soil P losses under conditions applicable to Ontario agriculture. The bulk soil samples and undisturbed soil columns were collected from sixty mineral soils and forty-four organic soils, covering major agricultural soils in Ontario. All surface soils were analyzed for Olsen P (Ontario’s agronomic STP method), Mehlich-3 P (M3-P), Bray-1 P (Bray-P), Fe-oxide coated filter paper strip P (FeO-P), water extractable P (WEP), and a single-point isotherm (PSI). The calculated degree of P saturation (DPS) included DPS_M3-1 (M3-P/(M3-P+PSI)), DPS_M3-2 (M3-P/(M3-Al+M3-Fe)), DPS_M3-3 (M3-P/M3-Al), DPS_Ol (Olsen-P/(Olsen-P+PSI)), DPS_Bray (Bray-P/(Bray-P+PSI)), and DPS_FeO (FeO-P/(FeO-P + PSI)). Amongst the tested soil P measures, soil WEP (r²=0.86-0.90) and FeO-P (r² = 0.82-0.85 for mineral soils and 0.57-0.71 for organic soils) were consistently significantly related with DRP concentration in surface runoff and leaching water across mineral and organic soils. In comparison, soil Olsen P (r² = 0.67-0.72), M3-P (r² = 0.74-0.84), DPS_M3-2 (r² = 0.85-0.90), and DPS_M3-3 (r² = 0.85-0.89) were significantly related with DRP concentration from mineral soils, but not (i.e. Olsen, Melich-3) or only weakly (i.e. DPS_M3-2 &-3) related with DRP concentration from organic soils. In addition, the DPS indices calculated by integrating STP with soil PSI were all significantly related with DRP concentration in surface runoff and leaching water, with r² = 0.77-0.82 for mineral soils and 0.61-0.88 for organic soils. Summarily, soil FeO-P showed promising potential as an indicator of soil P losses for both mineral and organic soils.